Research and development are now progressing on wireless communication systems using OFDM (Orthogonal Frequency Division Multiplexing) for the purpose of high-speed, large-capacity wireless communication. In an OFDM-based system information is transmitted by using multiple orthogonal subcarriers with different frequencies. A transmitting side generates data to be transmitted for the frequency domain, converts the data into a signal for the time domain using IFFT (Inverse Fast Fourier Transform), and transmits a wireless signal. A receiving side converts the signal for the time domain into the signal for the frequency domain using FFT (Fast Fourier Transform) to retrieve the original information.
OFDMA (Orthogonal Frequency Division Multiple Access) provides multiple accesses by assigning subcarriers to multiple terminals for OFDM-based multiplexing.
LTE is representative of cellular wireless communication standards using OFDMA. According to the LTE, a base station assigns a frequency resource in units of the specified number of subcarriers to a predetermined time interval called a subframe corresponding to each of uplink and downlink signals.
The LTE uses an adaptive modulation technique. The technique defines multiple types of MCS (Modulation and Coding Scheme), which is modulation scheme for data packet transmission and an encoding rate for error correction codes, and selects candidates most suitable for the channel state. The base station also makes these determinations.
When a downlink data packet is transmitted, a subframe same as the data packet is used to transmit downlink control information including a frequency resource used for the data packet transmission and MCS applied to the data packet.
A terminal checks each subframe for the presence of the downlink control information targeted for the terminal itself.
When there is the downlink control information targeted for the terminal itself, the terminal references the frequency resource specified in the downlink control information and uses the MCS specified in the downlink control information to decode the received data packet.
When there is no downlink control information targeted for the terminal itself, the terminal does not perform a downlink reception process on that subframe.
As a result of decoding, the terminal transmits ACK for successful decoding or NAK for unsuccessful decoding to the base station after four subframes for the downlink data packet reception.
When notified of NAK from the terminal, the base station retransmits the data packet using HARQ (Hybrid Automatic Repeat Request).
In HARQ, a packet is divided into sub-packets to transmit successively, and received power during the retransmission is added to the accumulation of previously received power, and redundancy bits of an error correction code are added. By using the aforementioned process, the probability of successful decoding is improved as the number of retransmissions is increased.
LTE includes the concept called an HARQ process so as to manage retransmission of respective data packets. The base station can manage up to eight HARQ processes per terminal.
After transmission of a data packet, the base station can transmit the next data packet without needing to wait for ACK for the previous data packet.
LTE uses MIMO (Multiple-Input Multiple-Output) technology that spatially multiplexes a data packet and transmits the data packet using the same frequency and time resource to increase the transmission capacity.
Of the MIMO technology, Multiple Codeword MIMO (MCW-MIMO) technology spatially multiplexes multiple data packets. LTE supports spatial multiplexing of up to two data packets.
When MCW-MIMO is applied, a terminal is notified of the MCS equivalent to two data packets using one set of downlink control information.
A HARQ of the MCW-MIMO manages retransmission equivalent to two data packets as one HARQ process.